The development of inexpensive digital cameras has resulted in the incorporation of cameras in a number of other products. For example, cellular telephones and PDAs are commonly equipped with cameras. While the initial cameras provided with the devices were of limited resolution, recent improvements in CMOS imaging arrays have resulted in cameras with more than two million pixels for such applications.
Further improvements in resolution and cost for such cameras could be obtained if the size of the pixels in the imaging array could be reduced. The cost of the camera is directly related to the area of silicon occupied by the imaging array and the accompanying circuitry. The imaging array occupies the majority of this area. Hence, to increase the number of pixels or to decrease the cost of a camera with the current number of pixels, the area of silicon must be reduced. The area of silicon, in turn, is determined by the size of the pixels in the imaging array.
A typical CMOS imaging array includes a two-dimensional array of pixel sensors that is organized as a plurality of rows and columns of pixel sensors. Each pixel sensor measures the light intensity at a corresponding point in the image for light of a particular color. Each pixel sensor includes a photodiode that converts light to an electronic charge that is stored in the photodiode until the photodiode is readout. Each pixel also includes one or more transistors that are used to generate a signal that is proportional to the stored charge and to couple that signal to a corresponding bus during the readout process.
The area of the photodiode determines the light sensitivity of the pixel sensor, hence, modifications in the imaging array that reduce the size of the active area of the photodiode also reduce the light sensitivity of the array. Accordingly, schemes for reducing the pixel sensor without lowering the light sensitivity of the camera are of interest. For example, in one scheme, a number of photodiodes share the same charge-to-voltage converter to reduce the area of silicon devoted to processing circuitry as opposed to light conversion.
To reduce the area of each pixel sensor further, either the noise levels of the individual photodiodes must be reduced or the dead space around each photodiode must be reduced. In general, each photodiode is an implant of a first semiconductor type in a substrate of a second semiconductor type. The wells are spaced apart from one another. The space between the photodiodes is effectively dead space in that it neither efficiently collects the charge nor provides space for processing circuitry.
Similarly, all photodiodes exhibit a “dark” current. That is, even in the absence of light, charge accumulates in the photodiode at some rate. In practice, the photodiodes are reset just prior to an image being projected onto the imaging array to remove any accumulated charge. However, there is always some delay between the reset and the image exposure during which the charge from the dark current accumulates. In addition, the dark current continues to accumulate even in the presence of light from the exposure. Finally, the dark current accumulates from the time the shutter is closed on the camera to the time the pixels are read out. Hence, the dark current represents a lower limit in the light sensitivity of the array, since, as the light levels decrease, a point is reached at which the dark current is the size or greater than the “light” current.
Modern CMOS manufacturing uses shallow trench isolation (STI) technology to isolate individual transistors and photodiodes. The interface between STI and the photodiode sidewall is known to have the highest dark current generation rate. Hence, as the pixel area is reduced to decrease the size of the imaging area, the ratio of the dark current to light current increases.